Fixes bug where simulator would barf on empty circuit.

Author: dabacon

cirq/google/sim/xmon_simulator.py

@@ -200,10 +200,12 @@ def run_sweep(
                     qubits,
                     initial_state,
                     param_resolver)
+                step_result = None
                 for step_result in all_step_results:
                     for k, v in step_result.measurements.items():
                         measurements[k].append(np.array(v, dtype=bool))
-                final_states.append(step_result.state())
+                if step_result:
+                    final_states.append(step_result.state())
             trial_results.append(SimulatorTrialResult(
                 param_resolver,
                 repetitions,

cirq/google/sim/xmon_simulator_test.py

@@ -118,6 +118,13 @@ def test_run(scheduler):
     assert result.measurements == {'a': [False], 'b': [False]}
 
 
+@pytest.mark.parametrize('scheduler', SCHEDULERS)
+def test_run_empty_circuit(scheduler):
+    simulator = xmon_simulator.Simulator()
+    result = run(simulator, Circuit(), scheduler)
+    assert len(result.measurements) == 0
+
+
 @pytest.mark.parametrize('scheduler', SCHEDULERS)
 def test_run_state(scheduler):
     simulator = xmon_simulator.Simulator()